This invention relates to an electrically conductive paint and, more particularly, to the preparation of such a paint.
Spacecraft are subjected to a wide range of thermal environments during service. One side of the spacecraft may face the blackness of free space, while the other side faces the sun. Heat is radiated into free space to cool the spacecraft, but the spacecraft is heated intensively in direct sunlight.
Active and passive temperature control techniques are used to maintain the interior temperature of the spacecraft, which contains persons or sensitive instruments, within acceptable operating limits. Active temperature control usually involves machinery or electrical devices, such as electrical heaters, electrical coolers, and heat pipes. The present invention deals with passive temperature control, where no machinery or electrical devices are used.
An established approach to passive temperature control is the use of surface coatings, typically termed xe2x80x9cpaintsxe2x80x9d, on the external surface of the spacecraft. A white paint, for example, has a low solar absorptance, while a black paint has a high solar absorptance. The selective application of such paints to various elements of the spacecraft exterior greatly aids in controlling its temperature.
The paint must also dissipate electrostatic charges that develop on the external surface of the spacecraft, as well as provide passive thermal control. The charges would otherwise accumulate to cause arcing and possible damage to, or interference with, sensitive electronic equipment on or in the spacecraft. In order to dissipate electrostatic charge, the paint must be somewhat electrically conductive, with a surface-resistivity on the order of about 109 ohms per square or less.
There are white, electrically conductive paints known for spacecraft use. Examples are found in U.S. Pat. Nos. 5,807,909; 5,820,669; 6,099,637; and 6,124,378. These paints are highly successful in a number of applications. There is, however, always a desire to improve the performance and manufacturing technology of such specialty paints. The present invention provides such an improvement.
The present approach provides an improvement to the processing of electrically conductive, electrostatic-dissipative paints, particularly white paints. The improved processing yields significantly better performance than prior processing approaches, at no added cost and without modifying the chemical composition of the paint. The performance is improved in respect to the application and coverage properties of the paint, the electrical performance, and the resistance to radiation damage, while maintaining excellent optical properties.
In accordance with the invention, a method for preparing a paint comprises the step of preparing a pigment mixture by the steps of providing an electrically conductive paint pigment, providing a paint vehicle, and mixing and milling the paint pigment and the paint vehicle to form a pigment mixture. Thereafter, a liquid paint is prepared by the steps of providing an inorganic paint binder, and mixing and dispersing the pigment mixture in the paint binder, without substantial milling of the pigment mixture in contact with the paint binder. The liquid paint may be applied to a surface, such as a portion of a spacecraft, and dried to form a solid paint.
The approach is applicable to a number of electrically conductive paint pigments and paints made from those pigments. The preferred paint pigments have a composition of A[xAl (1xe2x88x92x)Ga]2O4(xcex4D), wherein A is selected from the group consisting of zinc, cadmium, and magnesium, D is a dopant selected from the group consisting of a cationic dopant having an ionic valence greater than+2 and an anionic dopant, the value of x is from 0 to 1, and the value of xcex4 is from 0 to about 0.2. The paint vehicle is preferably water. The inorganic paint binder is preferably potassium silicate.
A key feature of the present invention is that the inorganic paint binder is not milled with the pigment. In a prior approach, termed a xe2x80x9cone-stepxe2x80x9d approach, the paint pigment, paint vehicle, and inorganic paint binder were mixed together and milled together. In the present approach, termed a xe2x80x9ctwo-stepxe2x80x9d approach, the paint pigment and paint vehicle are milled together, and this milled mixture is thereafter mixed with and dispersed into the paint binder without substantial further milling of the paint pigment. It has been found that the milling of the paint pigment with the inorganic paint binder in the one-step process has an adverse effect on the properties of the paint, which are avoided in the two-step process.
Thus, it is preferred that the mixing and dispersing step, when the paint pigment is in contact with the inorganic paint binder, be performed without substantially reducing the particle size of the paint pigment. It has been determined that inorganic paint binders, particularly potassium silicate paint binders, may attack the surfaces of the particles of the paint pigment during mechanical milling, leading to adverse effects on the electrical properties and other properties of the final paint. To avoid such influences, any substantial reduction in the particle size of the paint pigment occurs during the mixing and milling step when the paint pigment is not contacted by the inorganic paint binder.
The present approach achieves significant improvements in the properties of the final paint by avoiding the milling of the paint pigment in the presence of the paint binder. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.